Method of spheroidizing steel stock



Sept. 25, 1956 o. E. CULLEN METHOD OF SPHEROIDIZING STEEL STOCK FiledJuly '22, 1954 I from Carbon Therma/ 'Cm'f/k-a/ Poi/1f;

Farce/72 Carbon we, Y /2 /:5 77/776 HO UK? V INVENTOR.- Orv/fie 15 CATTOFrNE United States Patent Oflice 511136011 Corporation, Toledo,Ohio, a corporation of Application July '22, 1954, Serial No. 445,016

7 claims, (Cl. 148-415 This invention relates to spheroidi'zi'ng steelstock, and more particularly, to a com aratively rapid method for'spheroidizirig cement'itiou's portions of steel stock.

In many instances, for example where severe forming operations must beperformed on steel stock, either car'- hon steel or alloy steel, it isnecessar to spheroidiz'e the cementitious portions thereof. YSphe'roidiZation has heretofore been achieved either by extremely longheating cycles where the work is first heated to a temperature withinthe austen'itiz'in'g range and then cooled slowly, or else cooled toabout room temperature, either by uenching or by slow cooling, and thenreheated to a temperature just below its Ac1 temperature, or the minimumtemperature at 'whieh, upon heatin g austenite is formed. In the formermethod, extremely long heating eyes are required, for example 80 to 170hours (depending on the size or the load and on the degree ofspheroidization required) in the ease of A. I. S. I. 1030 steel, whichis known to be hard to spheroidi'ze. The latter process is not generallyused unless spheroidiz'atio'n is to be carried out at an intermediaterocessing. point, since it involves a double heating operation, and itis generally equally long.

The resent invention is based upon the discovery that, by proper controlof'proeessing temperatures, spheroidization can be accomplished in afraction of the time required by former techniques.

It is therefore, an object of the invention to provide an improvedspheroidizlrig process.

I Other objects nd advantages will be ap arent from the descri tionwhich follows, reference being had to the attached drawing, or whichFig. l is a portion of the iron-carbon diagram showing thermal criticalpoints as a function of carbon content, and Fig. 2 is" a typicaltimetemperature curve according to this invention.

According to the invention an improvement is provided in a method trspheroidizing eetnehtitious ortions of steel tock which comprisessubstantially austeniti'zihg the stock by heating to a temperature atwhich sl'l'bstali tia-lly all eemehtite therein is converted toaustehi'te, and cooling the stock f om such tem erature to a temperaturesub tantially below the Air temperature for the stools. The im ro ementor the invention ons'ist 'i cooling he austenltize'd stock to atemperature below the Arr tem erature for that stock, but above about.

1600 1?. and preferabl not more than bout se F. below the Airtemperature for a time sulfic'i ntlylong that a part of the austenite isconverted to cementite nuclei, reheating the stock to a temperaturebetween its Ac temperature and a temperature not more than about l0?"below its Ari temperature and preferably abo e it's Arr; temperaturemaintaining the stock at'a sphetoldiz As a first step in spheroidiz'ingsteel according to the improved method of the invention it is necessaryto heat the stock to a temperature at which cementitious portionsthereof are substantially converted to austenite. In the case of ahypoeuteetoid steel this can be accomplished by heating above the A61temperature, or to a point at which ferrite and austenite coexist, or bycarrying the heating to a higher temperature where only austenite is thestable form. In the former instance little if any ehan e in position ofcementite occurs, so that spheroidi'zed cementite appears in thefinished stock in larger areas, but in approximately the same positionsthat the cementite deposits, in another form, were present in theoriginal stock. In the latter case, substantially the entire stockbecomes austenitic so that a redistribution of cementitious portions,after spheroidizing occurs. In the instance of hypereiltectioid steel,complete austenitizing occurs only upon heating above the Acmtemperatu'r'e, which is a function or carbon content. In practice it isoften referred to austenitize at temperatures somewhat below the Acmline, but siifiiciently' high to austenitiz'e substantially all thecementite. Since only ausfenite is the stable form at austeiiitiz'ingtemperatures above the Acm line for hypereu'tect'oid steel, it will beapparent that substantial migration or carbon,- or FeaC, and thereforeof the location of spheroidiied particles in the finished'pfodu'ct, canbe expected ill this type of steel. The preferred range ofaustenit'iz'ing temperatures as a function of carbon content isrepresented for unalloyed, or carbon steel, by the shaded ortion of theattached drawing.

As is above stated, in practicing the method of the invention, theaustenitized stock is cooled to a temperature below its Ari temperature,but not lower than about 1'00'0 F., as a second step. Although theinvention is not limited to the following theoretical explanation, it isbelieved that, upon cooling to a temperature within the indicated range,c'fiie'ntit nuelei are precipitated from the stock and, upon subsequentreheating at a temperature near the A61 temperature, additionalcementite precipitate'd from the austenitic phase f rms spherules aroundthese nuclei. In this connection it has been ob served that maintainingthe stock below the Ali tem erature for a longer poriod of time tends toproduce smaller spherules than does maintaining the stock at suchtemperature for arelatively short period of time. This difference isbelieved to be attributable to the formation, in the former instaue'e,ot a lar er number of nuelei around which the pherules form, as comparedwith a smaller nufnbef of nuclei around which larger spherllle's ffifnlin thelatter instance. It has also been observed that eementiteprecipitation which occurs during this step of thorn-edited or": theinvention should be terminated,- by reheating as hereinafter describedin more detail,- before thest'dck is converted to a relativelystabilized pearlitic form. If the" precipitation of cementite is allowedto proeeed, durin this stage, untilthe stock contains sllbst an tialamounts or pearlite, it is found that ubsequent mg temperature withinthe las -indicated ra ge fo a time stl'fiiient'to formspheroidizedeementite, and cooling the stock. A substantiall completelys heroidized micro: structure is usually achieved by virtue of the aboveros ess after from about four to about ten hours at the sphei'oidizingtemperature;

spheroidizationproceeds onl with substantial dihieulty 'tiiid at arelatively low rate. It is preferred that this c'e merititepreeipitation step Of the method of the invention be terminated b'fo'r'esubstantial equilibrium is reached, 61* While some austemte remains inthe stock; If is also preferred that the sto'ek be quenchedfrom theausteriitii ing temperature, or at least from between the A01 and theArt temperatures after austenitizln to the temperature at whichcementite preeipi-tated, asdescribed, because the presses can besubstantially speeded-up by virtue of such queil ilig since the timerequired for such cooling 'is es sefitially lost time,- and sinceprecipitation occurs more raoidly at comparatively low temperatures.However, as is hereinatter'deseribed in more detail, the entire splii'oidizing process according to the invention can be carried out in asingle continuous furnace, if desired.

The final step in sphero'idizing steel stock according to the inventionis reheating to a temperature between its A01 temperature and atemperature not more than about F. below its Ari temperature andpreferably above its Ari temperature, and maintaining the stock withinsuch range for from about four hours to about ten hours. When the stockhas been cooled, prior to this reheating, as described in the precedingparagraph, spheroidization proceeds rapidly at temperatures within suchrange, with the result that even hard to spheroidize stock issubstantially completely spheroidized within the indicated time.

The attached drawing shows one specific iron-carbon diagram whereon thedifferences between the various critical points upon heating and coolingare represented. It is 'known that differences between these criticalpoints are a function of the composition of the particular metal beingstudied. In consequence, in order to illustrate the various criticaltemperatures of all alloy steels, or even for all grades of carbonsteels, it would be necessary to prepare a separate diagram for eachsuch steel. However, it is generally true that the A1 change, which termis used herein and in the appended claims in its usual and acceptedsense, namely to indicate the minimum temperature at which austeniteforms or disappears as a. stable constituent of a steel, occurs at ahigher temperature upon heating (AC1) that upon cooling (Ari), for anygiven steel. A detailed discussion of the thermal critical points forsteel will be found in The Metallography and Heat Treatment of Iron andSteel, pages 110l28 of the 1926 edition, Albert Sauveur. The specifictemperatures shown on the attached drawing must be considered asexemplary only, however, because of the differences in these specifictemperatures which occur with difierences in composition.

If the particular temperatures, especially the AC1 and the Artemperatures, of a particular stock which it is desired to spheroidizeaccording to the invention are not known, they can readily be determinedaccording to procedures described in the Sauveur reference, cited above.In many cases, however, the temperatures can be approximated withsufiicient accuracy from the attached drawing.

The following example is presented solely for the purpose of furtherillustrating and disclosing the invention, and is in no way to beconsidered as a limitation thereon.

Example Samples of A. I. S. I. 1030 steel were continuously spheroidizedin a furnace schematically illustrated on the last page of a reprintentitled Continuous Short-Cycle Anneals for Spheroidization ofCartride-Case Steel, originally published in Metal Progress, July 1953.The Ac temperature of this stock was about 1340? 1 while the Aritemperature was about 1250 "F. The furnace controllers were setso thatthe stock was heated to about 1380 F. in approximately 3% hours in aninitial heating zone, maintained at about 1380" F. for approximately twohours in an austenitizing zone, cooled to about 1245 F. in about 1 /2hours and maintained at about 1245 F. for approximately hour in anucleating zone, reheated to about 1325 F. and maintained at suchtemperature for approximately eight hours in a spheroidizing zone, andthen cooled to about 1100 F. in approximately 2% hours in a finalcooling zone. The work was then discharged from the furnace. Samples,after'polishing and etching, were examined and found to showsubstantially completely spheroidized structures.

It is known that the displacement of the critical temperatures of steelsfrom the theoretical equilibrium critical temperatures as currentlypublished will vary both with the composition of the steel and the rateof heating or cooling. In Fig; 2 this has been illustrated by the curvesof the converging A01 and Ari lines. The superimposed annealing cyclecurve of Fig. 2 will infact be a wavy. line due to the practicalimpossibility of maintaining constant furnace temperatures. It has beendemonstrated that carbides, in the form of pearlite or cementite, willdissolve less readily into austenite than they precipitate therefrom,the classic demonstration of this being the carburization of steel to upto 3 /2 or so per cent carbon at about 1700 F. in less than one hour forthin steel strips, in a carburizing atmosphere whose carbon potential islimited by phase rule considerations to the Acm line, or about 1.2%carbon. Considering these facts, it is apparent that on'the sub-criticalannealing portion of this process, the apparent AC1 line for the steeltreated in a given furnace maybe higher than the text-book figures,since it is necessary only to periodically drop the work being annealedto below the AC1 critical temperature to cause the spheroidizing toproceed. In such a case the cementite which is agglomerated as spherulesbelow the AC1 line as the temperature wavers therabout will notsubstantially redissolve into the austenite during the moments the workis actually above the theoretical Ac1 line. For this reason theillustrated Aci and Ari lines may appear too widely spaced in Fig. 2,but in fact they represent a close approach to the practicaltemperatures with which one must work in industrial heating equipment.

it will be apparent that the method of the invention is not limited to aspheroidizing cycle carried out in a furnace or furnaces, as salt orother liquid bath-s could equally well be used for carrying out thevarious heating operations involved therein. It will also be apparentthat other changes and modifications can be made from the specificdetails disclosed and described without departing from the spirit of theattached claims.

I claim:

1. In a method for spheroidizing cementitious portions of steel stockwhich comprises austenitizing the stock by heating to a temperature atwhich cementite therein is converted to austenite, and cooling the stockfrom such temperature to a temperature below the Ari temperature for thestock, the improvement which consists in cooling the austenitized stockto a temperature below its Ar1 temperature, but not lower than about1000 P. for a time sufiiciently long that a part of the austenite isconverted to cementite nuclei, but not sufliciently long for theformation of a relatively stable pearl-itic structure, reheating thestock to a temperature between its A0 temperature and its Aritemperature, maintaining the stock at a temperature within thelast-indicated range for from about four hours to about ten hours, andcooling the stock, whereby aspheroidized microstructure is achieved. 1

2. In a method for spheroidizing cementitious portions of steel stockwhich comprises austenitizing the stock by heating to a temperature atwhich substantially all cementite therein is converted to 'austenite,and cooling the stock from such temperature to a temperature below theAri temperature for the stock, the improvement which consists in coolingthe austenitized stock to a temperature below its Ari temperature butabove about 1000 F. for a time sufficiently long that a part of theaustenite is converted to cementite nuclei, but not sufficiently longfor the formation of a relatively stable pearlitic structure, reheatingthe stock to a temperature between its AC1 temperature and a temperaturenot more than about 10 F. below its Ari temperature, maintaining thestock at a tempera-ture within the last-identified range for from aboutfour hours to about ten hours, and cooling the stock, whereby asubstantially completely spheroidized microstructure is achieved.

3. In a method for spheroidizing cementitious portions of steel stockwhich comprises austenitizing the stock by heating to a temperature atwhich substantially all cementite therein is converted to austenite, andcooling the stockfrom such temperature to a temperature below the Arttemperature for the stock, the improvement which consists in cooling theaustenitized stock to a temperature between the Art temperature and theAc1 temperature, rapidly cooling the stock to a temperature below itsAri temperature but above about 1000 F. for a time such that only aportion of the austenite is converted to cementite nuclei, reheating thestock to a temperature between its A01 temperature and a temperature notmore than about F. below its Ari temperature, maintaining the stock at atemperature within the last-identified range for from about four hoursto about ten hours, and cooling the stock, whereby a substantiallycompletely spheroidized microstructure is achieved.

4. In a method for spheroidizing cementitious portions of steel stockwhich comprises austenitizing the stock by heating to a temperature atwhich cementite therein is converted to austenite, and cooling the stockfrom such temperature to a temperature below the AH temperature for thestock, the improvement which consists in cooling the austenitized stockto a temperature below its All temperature, but not more than about 50F. therebelow, maintaining the stock at a temperature within thelast-identified range for a time sufficiently long that at least a partof the austenite is converted to cementite nuclei, reheating the stockto a temperature between its AC1 temperature and a temperature not morethan about 10 F. below its Ari temperature, maintaining the stock at atemperature within the last-above identified range until a substantiallycompletely spheroidized microstructure is achieved.

5. In a method for spheroidizing cementitious portions of steel stockwhich comprises austen-itizing the stock by heating to a temperature atwhich cementite therein is converted to austenite, and cooling the stockfrom such temperature to a temperature below the Art temperature for thestock, the improvement which consists in quenching the austenitizedstock to a temperature below its Ari temperature, but above about 1000F., maintaining the stock at a temperature Within such range for a timesuch that a part only of the austenite is converted to cementite nuclei,reheating the stock to a temperature between its A01 temperature and atemperature not more than about 10 F. below its Ari temperature,maintaining the stock at a temperature within the lastabove identifiedrange for from about four hours to about ten hours, and cooling thestock, whereby a substantially completely spheroidized microstructure isachieved.

6. In a method for spheroidizing cementitious portions of steel stockwhich comprises austenitizing the stock by heating to a temperature atwhich cementite therein is converted to austenite, and cooling the stockfrom such temperature to a temperature below its Ari temperature, theimprovement which consists in cooling the austenitized stock to atemperature below its Ari temperature, but not lower than about 1000 F.for a time suificiently long that at least a part of the austenite isconverted to cementite nuclei, reheating the stock to a temperaturebetween its AC1 temperature and a temperature not more than about 10 F.below its All ternperature, maintaining the stock at a temperaturewithin the last-indicated range until a substantially spheroidizedmicrostructure is achieved, and cooling the stock.

7. In a method for s'pheroidizing cementitious portions of steel stockwhich comprises austenitizing the stock by heating to a temperature atwhich cementite therein is converted to austenite, and cooling the stockfrom such temperature to a temperature below its Ari temperature, theimprovement which consists in cooling the austenitized stock to atemperature below its Ari temperature, but not lower than about 1000" Ffor a time sufficiently long that at least a part of the austenite isconverted to cementite nuclei, but not sufiiciently long for theformation of a relatively stabilized pearlitic structure, reheating thestock to a temperature between its A01 tempera ture and a temperaturenot more than about 10 F. below its Ari temperature, maintaining thestock at a temperature within the last-indicated range until asubstantially spheroidized microstructure is achieved, and cooling thestock.

References Cited in the file of this patent UNITED STATES PATENTS2,181,947 McCarroll Dec. 5, 1939 2,188,155 Payson Jan. 23, 19402,368,418 McCarroll Jan. 30, 1945 2,376,454 Schneider May 22, 1945

1. IN A METHOD FOR SPHEROIDIZING CEMENTITIOUS PORTION OF STEEL STOCKWHICH COMPRISES AUSTENITIZING THE STOCK BY HEATING TO A TEMPERATURE OFWHICH CEMENTIE THEREIN IS CONVERTED TO AUSTENITE, AND COOLING THE STOCKFROM SUCH TEMPERATURE TO A TEMPERATURE BELOW THE AR1 TEMPERATURE FOR THESTOCK, THE IMPROVEMENT WHICH CONSISTS IN COOLING THE AUSTENTITIZED STOCKTO A TEMPERATURE BELOW ITS AR1 TEMPERATURE, BUT NOT LOWER THAN ABOUT1000* F. FOR A TIME SUFFICIENTLY LONG THAT A PART OF THE AUSTENITE ISCONVERTED TO CEMENTITE NUCLEI, BUT NOT SUFFICIENTLY LONG FOR THEFORMATION OF A RELATIVELY STABLE PEARLITIC STRUCTURE, REHEATING THESTOCK TO A TEMPERATURE BETWEEN ITS AC1 TEMPERATURE AND ITS AR1TEMPERATURE, MAINTAINING THE STOCK AT A TEMPERATURE WITHIN THELAST-INDICATED RANGE FOR FROM ABOUT FOUR HOURS TO ABOUT TEN HOURS, ANDCOOLING THE STOCK, WHEREBY A SPHEROIDIZED MICROSTRUCTURE IS ACHIEVED.